At present, photolithography is widely employed in sub-micron resolution integrated circuit (IC) manufacturing. With an increasing degree in gold-bumping and other wafer-level IC packaging technologies, there has been an increasing demand for the projection optical system having a large depth of focus, a high throughput, and a relatively low (i.e., a few microns) resolution. Projection photolithography has been developed and applied gradually.
The present invention described below is related to the optical system described in U.S. Pat. No. 6,879,383 (hereinafter, “the '383 patent”) issued on Apr. 12, 2005 to Mercado and assigned to Ultratech, Inc.
FIG. 1 is a cross-sectional diagram of an example optical system 100 according to the '383 patent.
The invention described in the '383 patent is a large field, broad spectral band, color-corrected, anastigmatic projection optical system that projects an image of a pattern formed on a reticle onto a substrate.
The term “large field” means a field having a rectangular dimension of about 50 mm×100 mm or greater. Also, the term “broad spectral band” refers a spectral band that includes the g, h, i, spectral lines of mercury (i.e., 436 nm, 405 nm, 365 nm).
Optical system 100 includes, along an axis A1, a concave spherical mirror M with an associated aperture stop AS2. The mirror M includes an aperture AP on the optical axis. The aperture AP may be used, for example, to introduce light into the optical system for aligning of an object (e.g., a mask) with its image or inspecting the object. The optical system 100 is essentially symmetrical relative to an aperture stop AS2 so that the system 100 is initially corrected for coma, distortion, and lateral chromatic aberration. All of the spherical surfaces in optical system 100 are nearly concentric.
Optical system 100 further includes a field corrector lens group G with positive refractive power arranged along axis A1 adjacent to and spaced apart from mirror M. The lens group G includes a positive lens element L1 with surfaces S3 and S4. The positive lens L1 is plano-convex with surface S4 being convex and surface S3 being flat. The lens group G further includes adjacent lens element L1 and towards mirror M at least two negative lens L2 and L3. Lens L2 has surfaces S5 and S6 and Lens L3 has surfaces S7 and S8. Negative lenses L2 and L3 are meniscus type, with S6 and S8 being convex.
Adjacent lens group G is a first prism PA with surfaces S1A and S2A, and a second prism PB with surfaces S1B and S2B. The surface S1A faces an object plane OP2 and the surface SIB faces an image plane IP2. The object plane OP2 and the image plane IP2 are spaced apart from respective flat surfaces S1A and S1B by respective gaps WDA and WDB representing working distances. There is complete symmetry with respect to the aperture stop AS2, WDA=WDB. Although prisms PA and PB are not included in field corrector lens group G, these prisms play a role in the aberration correction, including chromatic aberration correction.
However, there is a certain angle between the object plane and the image plane. Because the object plane is not parallel to the image plane, there are some difficulties in photolithography machine design.